This invention relates generally to heat exchanger coils and, more particularly, to a method and apparatus for the belling and expanding of plate fin heat exchanger coils for dimension control.
A plate fin heat exchanger coil is commonly constructed with a plurality of flat, parallel plates having laterally spaced holes therein for receiving refrigerant tubes, or hairpin tubes, therein. At each end of the plate fin bundle, there is a tube sheet composed of heavier material, and adjacent one of the tube sheets, the open ends of the hairpin tubes are fluidly connected by way of U-shaped return bends that are secured thereto by way of soldering, brazing, or the like. When the coils are installed into a refrigeration system, the refrigerant is made to flow through the hairpin tubes, and the air to be cooled or heated is made to flow over the plate fins, such that a heat transfer is thereby affected.
As is known, the tube expansion process is generally carried out by passing tube expanding rods through the open ends of the hairpin tubes and then belling the tube. A backing plate is placed against the tube bends during compressive expansion to prevent the tubes from being driven out of the unit as the expanding tools are forced therethrough. As a result of this holding action the tubes are compressed rearwardly as they are being expanded outwardly by the tools. This in turn, causes the tubes to shrink so that the axial length of each tube can vary dramatically in final assembly. Because of the differences in tube length, belling of the tubes is difficult and generally results in uneven or misaligned bells being formed in the tube ends. The return bends therefore cannot be properly seated within the bells leading to the formation of relatively weak or incomplete solder or braze joints in this critical region. Further, the bell ends can also be uneven, which also makes automatic brazing or soldering difficult.
In order to better facilitate the formation of the tube bells and the joining of the return bends therein, it has been the common practice in the art to bring the open ends of the hairpins a considerable distance out from the adjacent tube sheet. The additional length of tube allows each bell to be brought to full depth without interference from the tube sheet. The unsupported length of tube between the bell and the tube sheet, however, represents the weakest section in the unit. Hydrostatic tests have shown that the flow circuit will generally rupture in this region when exposed to high internal stresses. Beyond weakening the unit the added length of tubing wastes costly material and thus raises the cost of each unit. Furthermore, the added tube length makes it difficult to compact the unit which in the case of a room air conditioner is of primary importance.
In order to improve the hydrostatic burst strength of a plate fin heat exchanger, a technique for tension expanding hairpin tubes into a fin pack unit, as explained in greater detail in U.S. Pat. No. 4,584,765 was developed. This prior technique was generally carried out on a three row coil which was first belled by a split collet and pin arrangement, and then expanded. Because of the closeness or the heat exchanger tube rows the bells that were formed are simple, single diameter bells, to which pre-tinned return bends are nested. Further, because of the closeness of the heat exchanger tube rows, the jaw members of the tube clamping fixture were relatively long, thin members. In operation, however, these long, thin jaw members bowed during the expansion process. Furthermore, the single diameter bell was not suitable for ultrasonic soldering, but had to use pretinned return bends.
Thus, to use ultrasonic soldering, the bell diameter would have had to be increased, which would have required even less space between adjacent tubes, making the jaw member even thinner and subject to more bowing in the horizontal direction.
Thus, there is a clear need for a simple apparatus for belling and tension expanding a plate fin coil which is suitable for ultrasonic soldering or autobrazing a return bend thereto.